Best practice standards such as IEEE documents and fire code state that you must deal with hydrogen in one of two ways: 1) Prove the hydrogen evolution of the battery (using IEEE 1635 / ASHRE 21), or 2) have continuous ventilation in the battery room. Vented Lead Acid Batteries (VLA) are always venting hydrogen through the flame arrester at the top of the battery and have increased hydrogen evolution during charge and discharge events.  Vented Lead Acid Batteries (VRLA) batteries are 95-99% recombinant normally, and only periodically vent small amounts of hydrogen and oxygen under normal operating conditions. However, both types of batteries will vent more hydrogen during equalize charging or abnormal charge conditions.

Vented Lead Acid (VLA) and vented Ni-Cad (Ni-Cad) batteries are either fully vented or partially recombinant battery types (Figure 1). They are batteries with free-flowing liquid electrolyte that allows any gasses generated from the battery during charging to be directly vented into the atmosphere. Partially recombinant batteries will contain a catalyst device in place of the flame arrestor and will result in half of the hydrogen released into the atmosphere.  In abnormal conditions, greater amounts of hydrogen gas will be released into the atmosphere.

Figure 1. VLA Cell Vented Lead Acid Battery

VRLA battery is designed to be a non-spillable, recombinant battery. Each cell is designed with a one-way pop-up valve that is incorporated into the container (jar) to prevent gas build up (Figure 2). During normal operation, this valve remains in the closed position, trapping the hydrogen gas allowing for the recombination process to reduce water loss in the battery. The valve will remain closed if the pressure remains under the manufacturer’s specification which is typically around 5 psi. However, the valve may open during recharge, equalize charge or any abnormal charge condition causing hydrogen to escape into the cabinet or room causing the potential for fire and/or explosion. This is caused by additional current that results in more recombination and heat generation that has the potential to lead to thermal runaway and greater hydrogen evolution of the battery.

Figure 2. VRLA Absorbed Glass Mat (AGM) Battery

Lithium batteries are non-aqueous electrolyte batteries (Figure 3). The cells are sealed and will not vent unless they have a thermal runaway event. During thermal runaway, considerable amounts of Carbon Dioxide, Carbon Monoxide, and Hydrogen. Depending on the Lithium battery chemistry, other combustible gasses such as Hydrogen Fluoride, methane, ethane, propylene, ethylene, etc. will be produced. Lithium-Ion batteries are equipped with a Battery Management (BMS) system that controls the state of charge and monitors battery parameters but cannot control a thermal runaway event resulting from an internal short.

Figure 3. Lithium Battery

• Ensure ventilation systems to remain on until gas detected is below 25% LEL

• Gas detection system shall have 2 hours of standby power in accordance with section 1203.2.5

• Failure of the gas detection system shall annunciate a trouble signal (NFPA 72) or audible and visible trouble signal at an approved constantly attended on-site location

Note: The State or Local Authorities Having Jurisdiction (AHJ) may require more than what is stated in the model codes.